Power control circuit employing semiconductor switching means responsive to the saturation of a magnetic amplifier



Oct. 5, 1965 J. L. HUTSON POWER CONTROL CIRCUIT 3,210,641 EMPLOYINGSEMICONDUCTO SWITCHING MEANS RESPO R NSIVE TO THE SATURATION OF AMAGNETIC AMPLIFIER Original Filed DEC. 19, 1961 DC Input Figl FigZ

INVENTOR Jeorld L.Hutson ATTORNEY United States Patent 19 Claims. (Cl.321-46 This is a division of application Serial No. 160,541 filedDecember 19, 1961. The present invention relates to apparatus forcontrolling the effective power applied to a load from a source ofalternating current supply voltage by controlling the conduction time ofa switching device connected in series with the load, and moreparticularly to such a power control circuit which utilizes a magneticamplifier as a control element.

It is often times desirable to control the power applied to a load froman alternating current supply source as a function of a relatively lowlevel signal. Feedback systems are exemplary of such applications. Also,in many instances it is desirable that the effective power applied tothe load from the A.C. supply source be controlled as a function ofseveral difierent variables.

Power control circuits which utilize magnetic amplifiers as a controlelement'are especially well suited for the above applications since itis normal to fiow DC. current through the control windings of a magneticamplifier and it is possible to have as many difierent control windingsas desired. Moreover, by providing different control windings it ispossible to achieve complete isolation between the various inputs.

The present invention provides an improved power control circuit forcontrolling the elfective power applied to a load as a function ofthecurrent flowing in the control winding of a magnetic amplifier. Inaccordance with the principles of the present invention, there isprovided a semiconductor switching means having first and secondterminals and means for connecting the switching means by the first andsecond terminals in series with a load and a source of alternatingcurrent supply voltage. The switching means is characterized by beingone which normally exhibits a high impedance between the two terminalsin at least one direction but which is switched to a quasi stable statein which the switching means exhibits a low impedance in the at leastone direction between the two terminals when a control signal is appliedtothe device. Once the switching means is switched to the quasi stablelow impedance state, it will remain there so long as holding currentflows through the two terminals. There is also provided a capacitor anda semiconductor diode switching means. The diode switching means isconnected to the capacitor for providing a discharge path for thecapacitor and applying to the first mentioned switching means a controlsignal responsive to the capacitor being charged to a voltage suificientto cause the diode switching means to switch to a low impedance state.There is also provided a magnetic amplifier connected for applying tothe capacitor energy to charge the capacitor to a voltage sufiicient tocause the diode switching means to switch to the low impedance stateresponsive to saturation of the core of the magnetic amplifier. Thecontrol winding of the magnetic amplifier is adapted to be connected toa control voltage source. The amount of current flowing through thecontrol Winding controls the time relationship between the beginning ofa half cycle of the alternating current supply voltage and the time atwhich the core of the magnetic amplifier becomes saturated,

thereby controlling the conduction time of the switching means and theeffective power applied to the load.

In accordance with the preferred embodiment of the invention, a secondcharge path for the capacitor is provided through resistive meansconnected to the alternating current supply voltage, and means isprovided for limiting the voltage to which the capacitor can be chargedfrom the second charge path during any half cycle to a voltage less thanthe breakover voltage of the diode switching means. Also, in accordancewith the preferred embodiment of the invention, the gate winding of themagnetic amplifier is connected between the above mentioned capacitorand a second capacitor, with the second capacitor being charged througha resistive path from the alternating current supply voltage. Themagnetic amplifier becomes saturated when the difference in the voltageto which the two capacitors are charged become sufiiciently great. Byutilizing the circuit of the preferred embodiment of the invention, itis practical to substantially reduce the demands placed upon themagnetic amplifier insofar as they relate to the amount of energytransferred through the gate winding and the amount of voltage that themagnetic amplifier must be able to withstand across its gate windingwithout becoming saturated.

The features of the invention which are believed novel are set forthwith greater particularity in the appended claims. Many objects andadvantages of the invention will, however, become apparent to thoseskilled in the art as the following detailed description of a preferredembodiment of the same unfolds when taken in conjunction with theappended drawing in which:

FIGURE 1 is a schematic diagram illustrating a preferred embodiment ofthe present invention; and

FIGURE 2 is a cure illustrating the voltage-current characteristics of apreferred type of device utilized in practicing the present invention.

Turning now to FIGURE 1 of the drawing, in accordance with the preferredembodiment of the present invention, one side of a source of alternatingcurrent supply voltage, suitably volts, 60 cycles, is connected throughline 10 to one side of a load 12. The other side of the load 12 isconnected through device Q and the output terminals of an autotransformer T to the other side of the supply voltage source, which issuitably grounded.

The device Q is suitably a symmetrical diode switching device whosevoltage current characteristics are as shown in FIGURE 2. Thus, thedevice Q will normally exhibit a high impedance to the flow of currentin either direction. However, when the voltage impressed across thedevice becomes greater than the avalanche voltage of the device, thedevice will switch to a quasistable, low impedance state and permit theflow of current in one of the two directions, the direction of currentflow being dependent upon the polarity .of the applied voltage. Thedevice will thereafter remain in the low impedance state until thecurrent flowing through the device falls below the holding current forthe device. Although a single device which exhibits a symmetricalswitching action, that is a device which can be switched to permit theflow of current in either direction, is preferred, two oppositely poledasymmetrical switching devices connected in parallel can obviously beutilized. Also, the principles of the present invention canadvantageously be utilized for controlling the conduction time of gatedtype switching devices such as the silicon controlled rectifier.

The other side of the load 12 is also connected through line 13 to oneside of each of resistors R and R The under terminal of resistor R isconnected through capacitor C to ground. Load 12 and resistor Rtherefore provide a charging path for the capacitor C with the charge onthe capacitor C being a function of the polarity of the half cycle ofalternating current supply voltage and the resistance of the chargingpath.

The under terminal of the resistor R is connected through resistor R andcapacitor C to ground. The juncture 14 between resistor R and resistor Ris connected through a double anode zener diode 16 to ground. Resistor Rfunctions as a voltage dropping resistor to limit the amount of currentflowing through the zener diode 16. It will be observed that the zenerdiode 16 limits the potential appearing at juncture point 14 to thezener voltage of the double anode zener diode 16. The capacitor C willbe charged toward the zener voltage of the device 16 through a chargepath comprising the resistor R The maximum voltage to which thecapacitor C will be charged as a result of the flow of energy throughthe charge path comprising resistor R will be not greater than the zenervoltage of the device 16.

The juncture point 18 between the resistor R and capacitor C isconnected through diode device Q and the input 20 of the transformer Tto ground. The device Q is suitably a symmetrical type device havingbreakover characteristics as shown in FIGURE 2. However, two oppositelypoled asymmetrical diode devices can be used.

When the charge on the capacitor C becomes equal to the breakovervoltage of the device Q the device Q will switch to its low impedancestate, providing a low resistance discharge path for the capacitorthrough the input 20 of the transformer T The discharge of the capacitorC through the device Q and input 20 will result in a voltage pulse beinginduced in the transformer T of a character to cause the device Q toswitch to its low impedance state.

The breakover voltage of the device Q is suitably greater than the zenervoltage of the device 16. The capacitor C will therefore not be chargedthrough resistor R to a voltage sufficiently great to cause the device Qto switch to its low impedance state. However, by properly choosing theresistance of the resistor R it will be possible to operate the systemunder conditions wherein the breakover voltage of device Q is less thanthe zener voltage of the device 16.

Juncture point 18 between resistor R and capacitor C is also connectedthrough the gate winding 22 of a magnetic amplifier 24 to the juncturepoint 26 between resistor R and capacitor C The control winding 28 ofthe magnetic amplifier 24 is suitably connected to a variable DC. powersupply (not shown). As is Well known in the art, the inductive impedanceof the gate Winding 22 will be quite large when the core of the magneticamplifier is unsaturated. However, when the potential between juncturepoint 18 and juncture point 26 becomes sufliciently high, the core ofthe magnetic amplifier will become saturated. The inductive impedance ofthe magnetic amplifier 24 will, upon saturation, become quite small andthe impedance between juncture point 18 and juncture point 26 will dropto a very low value. Upon this occurrence, capacitor C will discharge,tending to charge the capacitor Subsequent to a saturation of thesaturable reactor 24, both the capacitor C and the capacitor C will alsobe charged through the charge path comprising the resistor R When thecharge on capacitor C becomes equal to the breakover voltage of thedevice Q device Q will switch to its low impedance state, providing adischarge path for the capacitor C through the input 20 of transformer TThe discharge of the capacitor C will result in a voltage appearing atthe output terminals of transformer T the voltage being of a characterto cause the device Q to switch to its low impedance state. Capacitor Cis suitably connected in shunt with device Q and the output terminals oftransformer T for providing the desirable filtering action and assistingin application of the control signal to device Q as described in thecopending application Serial No. 160,541.

It will be noted that utilizing the above described arrangement, themagnetic amplifier 24 must only withstand the difference in voltagebetween juncture points 18 and 26. Also, since the capacitor C is atleast partially charged to a potential established by the zener diode16, in many instances the amount of charge which must be transferredfrom capacitor C to capacitor C to accomplish the firing of the device Qcan be quite small, reducing the demands placed on the magneticamplifier 24 and'resulting in faster response in the action of thecircuit.

It is very desirable that the magnetic amplifier 24 becomes saturatedeach half cycle, otherwise erratic firing and erratic control willresult due to the charge left in the capacitor C and the capacitor Cwhich was not dissipated before going into the succeeding half cycle.The components R R R C C Q and device 16 are therefore preferably chosensuch that the magnetic amplifier 24 will saturate near the end of thehalf cycle, for example, at a phase angle of approximately when thecurrent flowing through the control winding 28 is adjusted for minimumpower out.

As is shown in the art, the voltage which must be impressed across thegate winding 26 to produce saturation of the magnetic amplifier 24 isdependent upon the magnitude of the current flowing in the controlwinding 28. Thus, the greater the current flowing in the controlwinding, the lower the voltage across the gate winding must be toproduce saturation. Accordingly, as the amount of direct current flowingin the control winding 28 is increased, the magnetic amplifier 24 willbecome saturated at an earlier point in the half cycle of appliedalternating current supply voltage, resulting in an increase in theeffective power applied to the load.

From the above, it will be seen that the present invention provides animproved power control system that comprises a semiconductor switchingmeans'having at least two terminals and means for connecting saidswitching means by said two terminals in series with a load and a sourceof alternating current supply voltage. The semiconductor switching meansnormally exhibits a high impedance between the two terminals, but whichis switched to exhibit a low impedance between the terminals in at leastone direction when a control signal is applied thereto, There is alsoprovided a capacitor and a diode switching device. The diode switchingdevice normally exhibits a high impedance to the flow of current but isswitched to a low impedance state when the charge on the capacitorbecomes equal to the breakover voltage of the diode switching device.One terminal of the diode switching device is connected to thecapacitor, the other terminal of the diode switching device beingconnected to apply to the switching means a control signal to cause theswitching means to switch to the low impedance state responsive todischarge of the capacitor through the diode switching device. There isalso provided a saturable reactor having a gate winding connected toprovide a charging path for the capacitor from a source of alternatingcurrent supply voltage. The time in the half cycle of alternatingcurrent supply voltage at which the saturable reactor saturates,permitting the capacitor to become charged to a voltage equal to thebreakover voltage of the diode device, is a function of the directcurrent flowing in the gate winding of the saturable reactor.

In accordance with the preferred embodiment of the invention, in orderthat the demands on the saturable reactor may be reduced, there is alsoprovided means for charging the first capacitor to a voltage less thanthe breakover voltage of the diode device and asecond capacitorconnected to be charged to a voltage greater than the breakover voltageof the device, with a saturable reactor being connected between thefirst and second capacitors.

Although the invention has been described only with regard to aparticular preferred embodiment thereof,

many changes and modificaions will become obvious to those skilled inthe art. The foregoing description is therefore intended to beillustrative and not limiting of the invention defined in the appendedclaims.

What I claim is:

1. A power control circuit comprising:

(a) semiconductor switching means having first and second terminals,said switching means normally exhibiting a high impedance between saidfirst and second terminals in at least one direction and being switchedto a quasi stable state wherein said device exhibits a low impedance insaid at least one direction between said first and second terminalsresponsive to a control signal being applied to said switching means andremaining in said quasi stable state so long as holding current flowsthrough said first and second terminals;

(b) means for connecting said switching means by said first and secondterminals in series with a load and a source of alternating currentsupply voltage;

(c) a capacitor;

((1) switching means connected to said capacitor for providing adischarge path for said capacitor responsive to said capacitor beingcharged to a voltage sufiicient to cause said switching means connectedthereto to switch to a low impedance state;

(e) means effective responsive to discharge of said capacitor throughsaid last mentioned switching means for applying to said first mentionedswitching means a control signal to cause said first mentioned switchingmeans to switch the low impedance state;

(f) a magnetic amplifier connected to apply to said capacitor energy tocharge said capacitor to said voltage sufiicient to cause said lastmentioned switching means to switch to the low impedance stateresponsive to saturation of said magnetic amplifier; and

(g) means for controlling the time relationship between the beginning ofthe half cycle of alternating current supply voltage and the time atwhich the magnetic amplifier saturates to thereby control the conductiontime of said switchingmeans and the effective power applied to saidload.

2. A power control circuit comprising:

(a) semiconductor switching means having first and second terminals,said switching means normally exhibiting a high impedance between saidfirst and second terminals in at least one direction and being switchedto a quasi stable state wherein said device exhibits a low impedance insaid at least one direction between said first and second terminalsresponsive to a control signal being applied to said switching means andremaining in said quasi stable state so long as holding current flowsthrough said first and second terminals;

(b) means for connecting said switching means by said first and secondterminals in series with a load and a source of alternating currentsupply voltage;

(c) a capacitor;

(d) switching means connected to said capacitor for providing adischarge path for said capacitor responsive to said capacitor beingcharged to a voltage sufficient to cause said switching means connectedthereto to switch to a low impedance state;

(e) means effective responsive to discharge of said capacitor throughsaid last mentioned switching means for applying to said first mentionedswitching means a control signal to cause said first mentioned switchingmeans to switch to the low impedance state;

(f) a magnetic amplifier having a gate winding connected to apply tosaid capacitor energy to charge said capacitor to said voltagesufficient to cause said last mentioned switching means to switch to thelow '6 impedance state responsive to saturation of said magneticamplifier; and

(g) means for connecting a control winding of said magnetic amplifier toa power supply whereby the time relationship between the beginning of ahalf cycle of alternating current supply voltage and the time at whichsaid magnetic amplifier saturates is controlled as a function of thecurrent flowing through said control winding from said power supply tothereby control the conduction time of said first diode switching deviceand the effective power applied to said load.

3. A power control circuit comprising:

(a) a semiconductor switching device having first and second terminals,said device normally exhibiting a high impedance between said first andsecond terminals in at least one direction and being switched to a quasistable state in which said device exhibits a low impedance in said atleast one direction between said first and second terminals responsiveto a control signal being applied to said switching device and remainingin said quasi stable state so long as holding current flows through saidfirst and second terminals;

(b) means for connecting said switching device by said first and secondterminals in series with a load and a source of alternating currentsupply voltage;

(c) a capacitor;

(d) a diode switching device connected to said capacitor for providing adischarge path for said capacitor and applying to said first mentionedswitching device said control signal responsive to said capacitor beingcharged to a voltage equal to the breakover voltage of said diodeswitching device;

(e) a magnetic amplifier connected for applying to said capacitor energyto charge said capacitor to the breakover voltage of said deviceresponsive to saturation of said magnetic amplifier; and

(f) means for controlling the time relationship between the beginning ofthe half cycle of alternating current supply voltage and the time atwhich said magnetic amplifier saturates to thereby control theconduction time of said first mentioned switching means and theeffective power applied to said load.

4. A power control circuit comprising:

(a) a semiconductor switching device having first and second terminals,said device normally exhibiting a high impedance between said first andsecond terminals in at least one direction and being switched to a quasistable state wherein said device exhibits a low impedance in said atleast one direction between said'first and second terminals responsiveto control signal being applied thereto and remaining in said quasistable state so long as holding current fiows through said first andsecond terminals;

(b) means for connecting said switching device by said first and secondterminals in series with a load and a source of alternating currentsupply voltage;

(0) a capacitor;

(d) a diode switching device connected to said capacitor for providing adischarge path for said capacitor and applying to said first mentionedswitching device said control signal responsive to said capacitor beingcharged to a voltage sufficient to cause said switching device to switchto a low impedance state;

(e) a magnetic amplifier having a gate winding and a control windingwound on a magnetic core;

(f) means connecting the gate winding of said magnetic amplifier toapply to said capacitor energy to charge said capacitor to a breakovervoltage of said diode switching device responsive to saturation of saidmagnetic amplifier; and

(g) means for connecting the control winding of said magnetic amplifierto a power source whereby the timc relationship between the beginning ofa half cycle of alternating current supply voltage and the time at whichsaid magnetic amplifier saturates is controlled as a function of thecurrent flowing through the control winding of said magnetic amplifierfrom said power source.

5. A power control circuit comprising:

(a) first and second diode switching devices each having first andsecond terminals, said devices normally exhibiting a high impedancebetween said first and second terminals in at least one direction andbeing switched to a quasi stable state in which said devices exhibit alow impedance in said at least one direction between said first andsecond terminals responsive to the voltage across the devices becomingequal to the breakover voltage of the devices and remaining in saidquasi stable state so long as holding current flows through said firstand second terminals;

(b) means for connecting said first device by said first and secondterminals in series with a load and a source of alternating currentsupply voltage;

(c) said first diode switching device being characterized by a breakovervoltage greater than the maximum instantaneous voltage of the appliedalternating current supply voltage and said second diode switchingdevice being characterized by a breakover voltage substantially lessthan the maximum instantaneous voltage of said alternating currentsupply voltage;

((1) a capacitor;

(e) means connecting said second diode switching device to provide adischarge path for said capacitor responsive to said capacitor beingcharged to a voltage equal to the breakover voltage of said second diodeswitching device;

(f) means effective responsive to discharge of said capacitor throughsaid second diode switching device for applying to said first diodeswitching device a voltage at least equal to the breakover voltage ofsaid first diode switching device;

(g) a magnetic amplifier connected for applying to 'said capacitorenergy to charge said capacitor to the breakover voltage of said seconddiode switching device responsive to saturation of said magneticamplifier; and f (h) means for controlling the time relationship betweenthe beginning of a half cycle of supply voltage and the time at whichsaid magnetic amplifier saturates to thereby control the conduction timeof said switching means and the efiective power applied to said load.

6. A power control circuit comprising (a) first and second diodeswitching devices each having first and second terminals, said devicesnormally exhibiting a high impedance between said first and secondterminals in at least one direction and being switched to a quasi stablestate in which said devices exhibit a low impedance in said at least onedirection between said first and second terminals responsive to thevoltage across the devices becoming equal to the breakover voltage ofthe devices and remaining in said quasi stable state so long as holdingcurrent flows through said first and second terminals;

(b) means for connecting said first device by said first and secondterminals in series with a load and a source of alternating currentsupply voltage;

(c) said first diode switching device being characterized by abreakover'voltage greater than the maximum instantaneous voltage of theapplied alternating current supply voltage and said second diodeswitching device being characterized by a breakover voltagesubstantially less than the maximum instantaneous voltage of saidalternating current supply voltage;

(d) a capacitor;

, (e) means connecting said second diode switching device to provide adischarge path for said capacitor responsive to said capacitor beingcharged to a voltage equal to the breakover voltage of said second diodeswitching device;

(f) means effective responsive to discharge of said capacitor throughsaid second diode switching device for applying to said first diodeswitching device a voltage at least equal to the breakover voltage ofsaid first diode switching device;

(g) a magnetic amplifier connected for applying to said capacitor energyto charge said capacitor to the breakover voltage of said second diodeswitching device responsive to saturation of said magnetic amplifier;and w (h) means for connecting a control winding of said magneticamplifier to a power supply whereby the time relationship between thebeginning of a half cycle of alternating current supply voltage and thetime at which said magnetic amplifier saturates is controlled as afunction of the current flowing through said control winding from saidpower supply to thereby control the conduction time of said first diodeswitching deviceand the effective power applied to said load.

7. A power control circuit as defined in claim 6 further including meanseffective for charging said capacitor to a voltage less than thebreakover voltage of said second diode device.

8. A power control circuit as defined in claim 6 further including asecond capacitor, means including a first resistor connected to providea charge path from said supply voltage for charging said secondcapacitor to a voltage in excess of the breakover voltage of said seconddiode device, means including a second resistor connected to provide acharge path from said supply voltage for charging the first mentionedcapacitor to a voltage less than the breakover voltage of said seconddiode device, and means connecting a gate winding of said magneticamplifier between said first mentioned capacitor and said secondcapacitor.

9. A power control circuit as defined in claim 8 further including azener diode having a zener voltage less than the breakover voltage ofsaid second diode device connected in shunt with said second resistorand said first mentioned capacitor.

10. A power control circuit as defined in claim 9 further including atransformer having an input and an output, means connecting the input ofsaid transformer in a series loop with first mentioned capacitor and thesecond diode device, and means connecting the output of said transformerto apply to the first diode device a voltage in excess of the breakovervoltage of the first diode device responsive to the discharge of thefirst mentioned capacitor through the input of the transformer ,uponsaid second .diode .device [switching to the low impedance state.

11. A power control circuit as defined in claim 10 wherein the output ofsaid transformer is connected in series with the first diode device.

12. A power control circuit as defined in claim 11 wherein said firstand second diode devices are symmetrical in their switching action.

13. A power control circuit as defined in claim 4 further includingmeans eifective for charging said capacitor to a' voltage less than thebreakover voltage of said diode switching device.

14. A power control circuit as defined in claim 4 "further including asecond capacitor, means including a -first resistor connected to providea charge path from said supply voltage for charging said secondcapacitor to .a voltage in excess of the breakover voltage of said diodeswitching device, means including a second resistor connected to providea charge path from said supply voltage for charging the first mentionedcapacitor to a voltage less than'the breakover voltage of said diodeswitching device, and means connecting a gate winding of said magneticamplifier between said first mentioned capacitor and said secondcapacitor.

15. A power control circuit as defined in claim 14 further including azener diode having a zener voltage less than the breakover voltage ofsaid diode switching device connected in shunt with said first mentionedcapacitor.

16. A power control circuit as defined in claim 15 further including atransformer having an input and output, means connecting the input ofsaid transformer in a series loop with the first metioned capacitor andthe diode switching device, and means connecting the output of saidtransformer to apply to the semiconductor switching device a voltage inexcess of the breakover voltage of the semiconductor switching deviceresponsive to the discharge of the first mentioned capacitor through theinput of the transformer upon said diode switching device switching tothe low impedance state.

17. A power control circuit as defined in claim 16 wherein thesemiconductor switching device is a second diode switching device andwherein the output of said transformer is connected in series with thesecond diode switching device.

18. A power control circuit as defined in claim 4 10 wherein saidsemiconductor switching device and said diode switching device aresymmetrical in their switching action.

19. A power control as defined in claim 17 wherein the breakover voltageof the first mentioned diode device is less than the maximuminstantaneous voltage of said supply voltage and the breakover voltageof said second diode device is greater than the maximum instantaneousvoltage of said supply voltage.

References Cited by the Examiner UNITED STATES PATENTS 2,910,641 10/59Boyer 321-66 2,998,547 8/61 Berman 315-200 3,005,946 10/61 Thompson323-22 3,034,015 5/62 Schultz 315-97 3,160,806 12/64 McDaniel et a1.321-47 3,188,490 6/65 Hotf et a1. 307-885 3,189,747 6/65 Hoff 307-32FOREIGN PATENTS 665,946 7/63 Canada.

LLOYD McCOLLUM, Primary Examiner..

1. A POWER CONTROL CIRCUIT COMPRISING: (A) SEMICONDUCTOR SWITCHING MEANSFIRST AND SECOND TERMINALS, SAID SWITCHING MEANS NORMALLY EXHIBITING AHIGH IMPEDINACE BETWEEN SAID FIRST AND SECOND TERMINALS IN AT LEAST ONEDIRECTION AND BEING SWITCHED TO A QUASI STABLE STATE WHEREIN SAID DEVICEEXHIBITS A LOW IMPEDANCE IN SAID AT LEAST ONE DIRECTION BETWEEN SAIDFIRST AND SECOND TERMINALS RESPONSIVE TO A CONTROL SIGNAL BEING APPLIEDTO SAID SITCHING MEANS AND REMAINING IN SAID EQUAL STABLE STATE SO LONGAS HOLDING CURRENT FLOWS THROUGH SAID FIRST AND SECOND TERMINALS; (B)MEANS FOR CONNECTING SAID SWITCHING MEANS BY SAID FIRST AND SECONDTERMINALS IN SERIES WITH A LOAD AND A SOURCE OF ALTERNATING CURRENTSUPPLY VOLTAGE; (C) A CAPACITOR; (D) SWITCHING MEANS CONNECTED TO SAIDCAPACITOR FOR PROVIDING A DISCHARGE PATH FOR SAID CAPACITOR RESPONSIVETO SAID CAPACITOR BEING CHARGED TO A VOLTAGE SUFFICIENT TO CAUSE SAIDSWITCHING MEANS CONNECTED THERETO TO SWITCH TO A LOW IMPEDANCE STATE;(E) MEANS EFFECTIVE RESPONSIVE TO DISCHARGE OF SAID CAPICITOR THROUGHSAID LAST MENTIONED SWITCHING MEANS FOR APPLYING TO SAID FIRST MENTIONEDSWITCH-